There are various types of compound semiconductor devices. Among them, a high electron mobility transistor (HEMT) can easily achieve a higher output as well as a higher frequency, and is used in an application such as a base station for mobile telephones by exploiting these characteristics.
In the HEMT, a two-dimensional electron gas induced in an electron transit layer serves as a carrier. There are various materials for the electron transit layer. For example, when a nitride semiconductor material such as GaN is used as the electron transit layer, a withstand voltage of the HEMT can be enhanced by a large bandgap inherent to the nitride semiconductor.
In order to induce the two-dimensional electron gas in a GaN layer of the electron transit layer, an AlGaN layer with different lattice constant than that of GaN layer and induces the polarization may be formed on the GaN layer. In this case, distortion is generated in the AlGaN layer due to the difference in lattice constant between the AlGaN layer and the GaN layer. Then, by piezoelectric polarization in the AlGaN layer generated due to this distortion and by spontaneous polarization of the AlGaN itself, it is possible to induce the two-dimensional electron gas in the GaN layer of the electron transit layer.
However, in the HEMT device, a short-channel effect occurs in some cases. The short-channel effect is a phenomenon in which threshold voltage decreases and a drain leakage current flows as well, when the gate length is made short. The short-channel effect is one of the factors that prevent the higher output of the HEMT.
Note that techniques related to the present application are disclosed in Japanese Laid-open Patent Publications No. 2013-206976 and No. 2013-74211.